The invention relates to a method for point-by-point and image-line-by-image-line recording of characters, particularly print characters, in a recording raster on a recording medium from stored character data. The character data of the characters to be recorded are called in and are converted into control instructions for a recording element. The invention also relates to an apparatus for the implementation of the method, the apparatus being referred to below as an electronic photocomposer.
An electronic photocomposer with an electron beam as the recording element is already known from U.S. Pat. No. 3,305,841, incorporated herein by reference. The characters required for a composing job are stored in a character memory in the form of font data. The text to be composed is converted into text data in a composition computer, the text data representing the composing instructions for the photocomposer. In a composing mode, the text data successively call the font data required for recording the text data from the character memory and the read-out font data are converted into a video signal which supplies the control instructions for the electron beam tube. Each character is recorded on the picture screen of the electron beam tube, and is composed of a plurality of juxtaposed vertical picture lines in a continuous line grating in the direction of the text lines. The characters are composed of black and white segments in accordance with their contours by means of a trace unblanking/blanking of the electron beam as controlled by the video signal.
The characters recorded on the picture screen of the electron beam tube are exposed on a recording medium by an optical system.
Given this known photocomposer, the characters of the individual alphabets must be stored for all occurring type sizes so that the capacity of the character memory is extremely high.
It is traditional to store the characters in the form of coded font data in order to reduce the capacity of the character memory. There are various coding methods for this purpose. For example, a point-by-point coding of characters is known from U.S. Pat. No. 3,305,841, incorporated herein by reference, a run length coding for the picture line segments is known from U.S. Pat. No. 3,480,943, incorporated herein by reference, and a coding of the contour lines of characters is known from German Pat. No. 29 19 013, incorporated herein by reference.
German Pat. No. 24 22 464 (corresponding to U.S. Pat. Re No. 30,679, incorporated herein by reference) already discloses a method for contour coding of characters and for conversion of the contour-coded font data into control instructions for the electron beam of an electron beam tube when recording or composing the characters. In this contour coding, the characters are described by pairs of contour lines in an XY coordinate system wherein the x-coordinates are modified in steps of equal length. Every pair of contour lines is defined by the y-coordinates of the starting points and by the slopes of the contour lines. The slopes are expressed as changes of the y-coordinates for successive x-coordinate steps. The curvature of a character contour is thus specified as a sequence of changing slopes. The characters are again constructed on the electron beam tube of juxtaposed image lines proceeding vertically to the text lines. Thus, every character is recorded in and of itself in the position prescribed by the line of text to be set or composed, and all characters of the text lines are successively recorded. The control instructions for switching the electron beam on and off are acquired by determining the respective intersections of the pairs of contour lines with the image line currently to be recorded, the electron beam being switched respectively on between an upper and a corresponding lower contour line.
The described way of acquiring the control instructions requires a relatively great computation time, so that a high recording speed is not achieved. Although a typographically high-quality of the photocomposing is achieved by employing an electron beam tube as the recording element, it is disadvantageous that the usable picture screen area of the electron beam tube and thus the recording format are limited. At present, there is a desire to compose entire newspaper pages in one work pass. In order to compose such a newspaper page in a photocomposer comprising an electron beam tube, a relative displacement between the picture screen area and the recording medium must be undertaken between individual sub-exposures having the size of the usable picture screen area. This, however, requires great mechanical expense, leads to positioning errors, and slows the recording speed.
European Patent application No. EP-A-0096079, incorporated herein by reference, already discloses a method and an apparatus for line-by-line and image-line-by-image-line recording of whole newspaper pages in a recording raster from coded font data according to a layout plan. For this purpose, a recording element, for example a mosaic recording element, is conducted picture-element-by-picture-element across the overall width of the entire newspaper page, so that respective full lines of the entire newspaper page are recorded. Every image line is composed of sub-image-lines which belong to different text blocks or text lines in accordance with the layout plan. The coded font data required for recording the individual sub-image-lines are called from a character memory and are decoded into control instructions. The control instructions of the individual sub-image-lines are ordered in accordance with the sequence of the individual sub-image-lines within the full image lines and are deposited picture-element-by-picture-element in the recording raster in a memory for the full newspaper page, so that the memory content reproduces the recordable information of the full newspaper page with picture element precision. During the recording of the full newspaper page, the individual control instructions are then output picture-element-by-picture-element and image-line-by-image-line, and are synchronized with the movement of the recording element across the recording medium, and are supplied to the recording element.
In order to save memory capacity, European Patent application No. EP-A-0096079, incorporated herein by reference, likewise already discloses that the full newspaper page be exposed in successive strips whose height covers a plurality of image lines and whose width corresponds to the width of the full newspaper page. For this strip-by-strip recording, only the coded font data belonging to the individual positions of the strip are respectively called in, edited into control instructions for the number of image lines corresponding to the height of the strip, and stored in a so-called window memory whose capacity corresponds only to the number of picture elements for one strip. Before the shift of the strip into a new position on the newspaper page, the control instructions are transferred by image lines from the window memory into a buffer memory having the same capacity and are read out from the buffer memory for the control of the recording element while the emptied window memory is re-filled. This memory principle, however, has the disadvantage that a transfer time for the control instructions must be established in every shift of the strip. This limits the recording speed. The specified method only functions faultlessly when the height of the window memory corresponds to the height of the largest character to be recorded. However, this condition which must be observed has the disadvantage that the required height of the window memory must always be re-defined on the basis of the character instructions when the text data are input and the memory controller must be correspondingly programmed. The maximum memory capacity must be selected such that the height condition is fulfilled even for the largest characters, for example in headlines on a newspaper page. A further disadvantage of the specified method is that the write-in of the control instructions into the window memory is particularly complicated and time-wasting when a character to be recorded does not lie fully in one strip (EP-A-No. 0096079, FIG. 12) or when two successive characters in a textline have different bottom line levels (EP-A-No. 0096079, FIG. 14). In these cases, control instructions for the first character must always be overwritten from sub-areas of the window memory into the buffer memory before these sub-areas can be occupied with control instructions for the following character. The publication contains no indications whatsoever as to how one should proceed when recording cursive characters and rotated text lines. It is also known to employ a deflectable laser beam as the recording element for recording such newspaper pages.